Speed sensitive on-demand torque coupling differential

ABSTRACT

An axle case assembly is provided with a speed sensitive torque coupling mechanism used to transmit torque from the ring gear to a planetary differential assembly. The differential assembly provides torque transfer proportional to the speed difference between the ring gear sub-assembly and a planetary gear set sub-assembly, wherein the invention splits a differential case assembly into two primary pieces and a speed sensitive mechanism is installed between each piece. The mechanism is entirely contained inside an axle differential case assembly. An optional limited slip device may be provided for the differential gears. The torque transmission coupling assembly eliminates the need for a center differential in the transfer case, i.e. an interaxle differential, thereby reducing the driveline complexity and cost without requiring a separate torque coupling in the transfer case or in-line with the driveline.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to a full time-four-wheel-drive vehicle inwhich front and rear wheels are always connected to each other through atorque transmission coupling.

b) Description of Related Art

Torque applied to a tire through a drive shaft propels a vehicle by thefriction between the tire and the surface of the road for the vehicle.Occasionally, slip takes place between the road surface and the tire.The ratio of the slip depends on the coefficient of friction between thetire and the road surface. The coefficient of friction fluctuates due tothe states of the road surface and the tire, the grounding load upon thetire, the magnitude of the torque transmitted to the tire, the drivingspeed of the vehicle, and so forth.

As for an ordinary two-wheel-drive vehicle, high torque is transmittedto each driving wheel through a transmission at the start of the vehicleso that a large slip could take place between the road surface and thetire of the wheel. The torque transmitted through the transmissiondecreases as the driving speed of the vehicle rises, so that the ratioof the slip falls.

When the torque transmitted to the tire is so high that the tire slips,the torque does not fully act to propel the vehicle, resulting in wastedmotive power, lowered fuel efficiency, and adverse vehicle handling.

When the fluctuation in the coefficient of friction is large or thecoefficient of friction is very small, as on a muddy road, a partiallyicy road, a snowy road, a graveled road, or the like, the stability ofmovement of the vehicle is reduced and the stopping distance increasesin the case of locking of the wheel in braking. Moreover, it issometimes difficult to maintain the direction of movement of the vehiclein the case of locking of the rear wheel (in particular, in braking).

For the above-mentioned reasons, four-wheel-drive vehicles have becomepopular vehicles for driving on a wide range of road conditions. Infour-wheel-drive vehicles, the driving power of an engine is dividedlytransmitted to four wheels to eliminate the above-mentioned drawbacksand problems.

Since a rotation speed difference arises between the front and rearwheels of the four-wheel-drive vehicle due to the turning radiusdifference between the front and the rear wheels at the time of turningof the vehicle, torsional torque is caused (a tight corner brakingphenomenon) between the drive shafts for the front and the rear wheelsif the turning is performed on a high-friction-coefficient road (such asa paved road), on which the driving wheel and the surface of the roadare less likely to slip relative to each other. For that reason,different types of four-wheel-drive vehicles have been developed inorder to prevent the deterioration of the moving property of eachvehicle due to the torsional torque, the increase in the wear of thetire, the shortening of the life of the vehicle, and so forth.

One of the different types of four-wheel-drive vehicles is a part timefour-wheel-drive vehicle in which the driver shifts from the four-wheeldrive mode to the two-wheel drive mode when running on ahigh-friction-coefficient road such as a paved road. Another type offour-wheel-drive vehicle is a full time-four-wheel-drive orall-wheel-drive vehicle equipped with a center differential unit fordividedly transmitting motive power to a front and a rear wheel driveshafts. Another type of four-wheel-drive vehicle is a fulltime-four-wheel-drive vehicle in which the front or rear wheels arealways driven and in which the rear or front wheels are driven through aviscous clutch which transmits torque by the viscosity of silicone oilor the like.

Although the part time-four-wheel-drive vehicle can be manufactured at arelatively low cost, it is troublesome to shift between the two-wheeldrive and the four-wheel drive and it is likely that the vehicle isslowly turned when the driver mistakenly fails to properly choosebetween four-wheel drive and two-wheel drive. It is less likely thatevery driver can precisely predict the occurrence of slip of the drivingwheel and take appropriate action.

Full time-four-wheel-drive vehicle, that are equipped with the centerdifferential unit, have a front wheel drive differential unit, whichdividedly transmits motive power to the right and left front wheels, anda rear wheel drive differential unit, which dividedly transmits motivepower to the right and left rear wheels. These full-timefour-wheel-drive vehicles suffer from a problem that no motive power istransmitted to any of the remaining three of four driving wheels whenone wheel is caused to spin or loses the tire grip due to overhanging onthe road side or ditch, a slip on an icy road, or the like. For thatreason, the center differential unit is provided with a differentiallocking mechanism. The differential locking mechanism is of themechanical type or the electronic control type. In the mechanical type,a differential rotation which takes place in the center differentialunit is stopped through manual shifting when no motive power istransmitted to the three of the four driving wheels in order to put thevehicle into the state of direct-connection four-wheel drive. In theelectronic control type, the speed of the vehicle, the angle of turningof the vehicle, the racing of the drive shaft, and so forth are detectedby sensors in order to put the differential locking mechanism into alocking or unlocking state through an electronic controller. As for themechanical type, it is difficult to set a differential locking starttime point, the time point cannot be changed depending on the movingcondition of the vehicle, and it is more difficult to automate thedifferential locking mechanism. As for the electronic control type, adevice for controlling the differential locking mechanism is morecomplex and the cost of production of the mechanism is very high.

Since the center differential unit comprises an input shaft whichreceives motive power transmitted from an engine through a transmission,a differential case connected to the input shaft, a pinion shaft whichis driven by the differential case, pinions rotatably attached to theperipheral surface of the pinion shaft, a first side gear which isengaged with the pinion and connected to a first differential means fordriving the front or rear wheels, a second side gear which is engagedwith the pinion and connected to a second differential means for drivingthe rear or front wheels, and the differential locking mechanism whichengages the differential case and the side gear with each other throughmechanical operation or electronic control, the cost of production ofthe center differential unit is very high and the weight of the vehicleis increased.

It is also known to replace the aforementioned center differential witha torque transmission coupling that includes an input shaft drivinglyconnected to the transmission and a first differential, an output shaftdrivingly connected to a second differential, an oil pump driven by therelative rotation between the input and the output shafts to generateoil pressure corresponding to the speed of the relative rotation, and afriction clutch mechanism engaging the input shaft and the output shaftwith each other by the oil pressure generated by the oil pump. Thetorque transmitted by the torque coupling is proportional to the speedof the relative rotation. When the rotation speed of the wheels drivenby the first differential is higher than that of the wheels driven bythe second differential, a rotation speed difference takes place betweenthe input and the output shafts. The oil pump generates the oil pressurecorresponding to that rotation speed difference. The oil pressure isapplied to the friction clutch mechanism so that torque is transmittedfrom the input shaft to the output shaft depending on the magnitude ofthe oil pressure. When torque is transmitted to the second differential,the rotation speed of the wheels drivingly connected to the seconddifferential is raised to approach that of the wheels driven by thefirst differential, thereby reducing the rotation speed differencebetween the input and the output shafts. In short, the torquetransmission coupling operates in response to the rotation speeddifference that takes place depending on the environmental situation ofthe vehicle and the moving conditions thereof. In other words, aprescribed slip is always allowed.

The conventional torque coupling assemblies, however, suffer fromdrawbacks inherent in their assembly and location within the vehicledrivetrain. Conventional torque coupling assemblies are installed in thetransfer case or in-line with the driveline or driveshaft.

The need therefore exists for a torque coupling assembly that eliminatesthe need for a center differential in the transfer case, i.e. aninteraxle differential, thereby reducing the driveline complexity andcost without requiring a separate torque coupling in the transfer caseor in-line with the driveline.

SUMMARY OF THE INVENTION

The present invention was made in consideration of the above-describedcircumstances. Accordingly, it is an object of the present invention toprovide a four-wheel-drive vehicle which does not have theabove-described drawbacks and problems and fulfills the functions offour-wheel drive under all conditions and whose constitution and costare simple and low, respectively.

The present invention provides an axle case assembly with a speedsensitive mechanism used to transmit torque from the ring gear to aplanetary differential housing. The inventive differential assemblyprovides torque transfer proportional to the speed difference betweenthe ring gear sub-assembly and a planetary gear set sub-assembly,wherein the invention splits a differential case assembly into twoprimary pieces and a speed sensitive mechanism is installed between eachpiece. In the preferred embodiment, the mechanism is entirely containedinside an axle differential assembly. An optional limited slip devicemay be provided for the differential gears.

The primary features, structures and objectives of the invention will bemore fully understood with reference to the following description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of a four-wheel-drive vehicle incorporatingthe torque coupling assembly of this invention.

FIG. 2 is a partially exploded view showing the ring gear assembly, thebearing assembly and the differential assembly of the invention with thespeed sensitive torque coupling device omitted.

FIG. 3 is a view of the torque coupling and differential assembly ofFIG. 2 shown in the assembled state with a pump system and clutch packinterposed between the ring gear sub-assembly and the differentialsub-assembly.

FIG. 4 is a perspective view of the axle differential case housing thetorque coupling assembly of this invention;

FIG. 5 is a side view of the axle differential and torque couplingassembly shown in FIG. 4;

FIG. 6 is a left-end view of the differential and torque couplingassembly shown in FIG. 5;

FIG. 7 is a sectional view of the differential and torque couplingassembly as viewed along section line VII--VII of FIG. 6;

FIG. 8 is a sectional view of the differential and torque couplingassembly as viewed along section line VIII--VIII of FIG. 6;

FIG. 9 is left end view of the differential and torque coupling assemblyas shown in FIG. 5 with components of the pump actuator shown in dottedlines;

FIGS. 10-13 are exploded views of the primary components of oneembodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to FIG. 1, the four-wheel-drive vehicle provided inaccordance with the present invention comprises an engine 110, atransmission 130 which is driven through a clutch 120 by the engine 110to change the speed of the output rotation of the engine 110. A transfercase 150 divides torque transmission between a first differential means140 that drives one of a front and a rear wheel systems and a seconddifferential means 170 that drives the other of the front and the rearwheel systems.

A torque transmission coupling 200 is provided between a ring gear and aplanetary differential housing. The torque transmission coupling 200comprises an oil pump that is driven by the relative rotation betweenthe ring gear sub-assembly and a planetary gear set sub-assembly togenerate oil pressure corresponding to the speed of the relativerotation. A friction clutch mechanism engages the ring gear sub-assemblyand the differential gear set sub-assembly with each other by the oilpressure generated by the oil pump. The torque transmission coupling hassuch a property that the torque transmitted by the coupling isproportional to the speed of the relative rotation.

With reference to FIGS. 2 and 3, the torque coupling assembly 200comprises a ring gear sub-assembly 210, a differential sub-assembly 220,and a bearing sub-assembly 230 (see FIG. 2). The ring gear subassembly210 includes a ring gear 212 fastened to a side case member 214 viafasteners 216. While FIG. 3 shows fasteners 216 in the form of boltsfixing the ring gear 212 to the side case member 214, it will beunderstood that various fastening assemblies may be employed withoutdeparting from the objectives of this invention. The differentialsub-assembly 220 comprises a differential case 222, a shaft 223 drivenby the rear differential case 222, pinions 224a, 224b rotatably attachedto the peripheral surface of the shaft 223, and side gears 226a, 226bengaged with the pinions 224. The side gears 226a, 226b drive the rightand left axles (not shown in FIG. 3).

Interposed between the ring gear sub-assembly 210 and the differentialsub-assembly 220 is a bearing assembly 230 which permits relativerotation between the ring gear sub-assembly 210 and the differentialsub-assembly 220.

Further provided between the ring gear sub-assembly 210 and thedifferential sub-assembly 220 is a speed-sensitive torque couplingassembly, shown generally as assembly 240. The speed-sensitive torquecoupling assembly 240 included in the preferred embodiment of thepresent invention comprises a fluid pump 250 and a clutch pack 260. Thefluid pump shown and described herein is a Gerotor type pump of theautomatically reversible unidirectional flow type. The specificstructure of the fluid pump 250 and clutch pack 260 will be describedbelow.

With the assembly of FIGS. 2 and 3, a torque coupling assembly isprovided within the axle differential case assembly. Therefore, when therotation speed of the wheels driven by the first differential 140 ishigher than that of the wheels driven by the second differential 170, arotation speed difference takes place. In that case, the pump 250generates the oil pressure corresponding to that rotation speeddifference. The oil pressure is applied to the friction clutch mechanism260 so that torque is properly distributed between the firstdifferential 140 and the second differential 170 depending on themagnitude of the oil pressure. When the torque is transmitted to thesecond differential 170, the rotation speed of the wheels drivinglyconnected to the second differential 170 is raised to approach that ofthe wheels driven by the first differential 140, thereby reducing therotation speed difference between the front and rear wheels.

In the low speed running of the vehicle, the absolute value of the speedof rotation transmitted to the first differential 140 is small, and therotation speed of the ring gear sub-assembly 210 is therefore small aswell. Even if the speed of the rotation of the differential sub-assembly220 output shaft is zero or very low, the absolute value of the rotationspeed difference between the sub-assemblies 210, 220 is small. Inaddition, the rising of the oil pressure generated by the oil pump atthe low rotation speed is generally slow due to the internal leak of thepump. For these reasons, the torque transmitted through the frictionclutch mechanism 260 is very low, so that the ring gear sub-assembly andthe differential sub-assembly are allowed to slip relative to eachother. As a result, torsional torque caused between a front and a rearwheel drive shafts at the time of slow turning of the vehicle isabsorbed by the friction clutch mechanism 260 to prevent a tight cornerbraking phenomenon.

In the high speed running of the vehicle, if the rotation speed of thewheels driven by the second differential 170 is even slightly lower thanthat of the wheels driven by the first differential 140, the absolutevalue of the rotation speed difference between the ring gearsub-assembly 210 and the differential sub-assembly is certain toincrease, because the absolute value of the speed of rotationtransmitted to the first differential 140 is large in proportion to thedriving speed of the vehicle. Therefore, the torque transmitted throughthe friction clutch mechanism 260 is also high, corresponding to theabsolute value of the rotation speed difference between the ring gearsub-assembly 210 and the differential sub-assembly 220 shafts so thatthese shafts are maintained in a torque transmission state approximateto a directly connected state. For that reason, in the rapid running ofthe vehicle, the torque of the engine is transmitted to the front andthe rear wheels, while the torque is divided nearly at a ratio of 50:50between them, so that the stability of the running of the vehicle andthe fuel efficiency thereof are enhanced.

Since the second differential 170 is always connected to the firstdifferential 140 through the torque transmission coupling 240, it is notnecessary to perform troublesome shifting between two-wheel drive andfour-wheel drive as is done in the conventional parttime-four-wheel-drive vehicle.

When some driving wheel slips during the running of the vehicle providedin accordance with the present invention, the rotation speed differencebetween the ring gear sub-assembly 210 and differential sub-assembly 220of the torque transmission coupling increases immediately so that theoil pressure corresponding to the rotation speed difference increases.Consequently, the friction clutch mechanism 260 immediately acts toprevent the increase in the rotation speed difference between the ringgear sub-assembly 210 and the differential sub-assembly 220 to keep theslipping driving wheel from skidding sideways. Excess torque istransmitted to the other non-slipping driving wheels instead of theslipping driving wheel, so that the torque of the engine transmittedthrough the transmission is dividedly transmitted to the first and thesecond differentials 140, 170. Appropriate driving forces are thusautomatically and constantly applied to the front and the rear drivingwheels with good response.

With reference to FIGS. 4-13, a specific embodiment of the inventionwill now be described. FIG. 4 is a perspective view of the preferredembodiment of the invention in an assembled state, and FIG. 4 shows thedifferential case 222 disposed within an outer housing 215 which isaffixed to the side housing 214. The bolt holes 214a pass through boththe side case member 214 and a flange 215a formed on the outer housing215. Apertures 214b are provided to receive bolts 216 to mount a ringger (not shown) to the side housing 214 and the outer housing 215. FIG.5 is a side view of the torque coupling differential of FIG. 4.

FIG. 6 is a left side view of the torque coupling differential of FIG. 5illustrating the side housing 214 and bolts 216 mounted within the boltholes 214b.

FIG. 7 is a cross sectional view of the torque coupling differential asviewed along section line VII--VII of FIG. 6, and FIG. 8 is a crosssectional view of the torque coupling differential as viewed alongsection line VIII--VIII of FIG. 6. FIG. 9 is left end view of thedifferential and torque coupling assembly as shown in FIG. 5 withcomponents of the pump actuator shown in dotted lines.

FIGS. 6-8 show the components of the torque coupling mechanism disposedbetween the ring gear sub-assembly 210 and the differential sub-assembly220. The ring gear 212 has been omitted from FIGS. 4-8. The differentialassembly shown in cross section includes the differential case 222,pinion gears 224a, 224b and side gears 226a, 226b, wherein the piniongears 224a, 224b are mounted on the shaft 223.

Disposed adjacent the side gear 226a is an inner clutch sleeve 242having external splines 242a. A clutch pack is disposed between the ringgear sub-assembly 210 and the differential sub-assembly 220. Forming theclutch pack are clutch plates 244 and 245 alternately mounted betweenthe inner clutch sleeve 242 and the outer housing 215. The clutch plates244 mate with the splines 215b formed on the clutch sleeve 242, and theclutch plates 245 mates with splines 215b formed on the inner surface ofthe outer housing 215. The clutch plates 244 frictionally engage theclutch plates 245 to form a torque coupling arrangement between the ringgear sub-assembly 210 and the differential sub-assembly 220. Torque istransferred from the ring gear to the outer housing 215, then to theclutch plates 245. The clutch plates 245 transmit torque to the clutchplates 244 which, in turn, transmit torque to the clutch sleeve 242. Theclutch sleeve 242 then transmits torque to the differential case 222.

A speed sensitive fluid pump arrangement 250 actuates the clutch pack toincrease the frictional engagement between the clutch plates 244, 245.The speed sensitive fluid pump arrangement 250 comprises an outer ringmember 252, an outer rotor 254 and an inner rotor 256. The inner rotor256 mates with the clutch sleeve 242, and the outer ring member 252mates with the ring gear sub-assembly 210 via pin 253.

As illustrated in FIG. 10, the inner rotor 256 has one less tooth thanthe outer rotor 254 and when the inner rotor 256 is driven it will drivethe outer rotor 254, which can freely rotate within the outer ringmember 252 thus providing a series of decreasing and increasing volumefluid pockets by means of which fluid pressure is created.

External to the pump itself, the inner rotor 256 is matingly connectedto the clutch sleeve 242, and the sleeve 242 meshes with clutch plates244. When relative motion takes place between ring gear sub-assembly 210and the differential sub-assembly 220, the clutch sleeve 242 will rotatethe inner rotor 256 of pump 250 to create fluid pressure.

The torque transmission coupling provided in accordance with the presentinvention comprises the ring gear sub-assembly 210, the differentialsub-assembly 220, the friction clutch mechanism 260, and the oil pump250 for engaging the clutch mechanism depending on the rotation speeddifference between the ring gear sub-assembly 210 and the differentialsub-assembly 220. Therefore, the torque coupling is an improvement overthe conventional center differential unit of the fulltime-four-wheel-drive vehicle, the cost of production of the coupling isvery low, and the weight of the four-wheel-drive vehicle provided inaccordance with the present invention is reduced.

When the front wheel system of the four-wheel-drive vehicle provided inaccordance with the present invention is driven by the firstdifferential means, torque is transmitted to the rear wheels at the sideof the second differential means as long as the front wheels are notlocked at the sharp braking of the vehicle. For that reason, ananti-locking effect is produced. In other words, the torque istransmitted to the rear wheels from the front wheels through the torquetransmission coupling. This serves to prevent the early locking of therear wheels, which would be likely to occur at the time of braking on alow-friction-coefficient road such as an icy road.

As described above, the four-wheel-drive vehicle provided in accordancewith the present invention fulfills the functions of four-wheel drivewell through the action of the compact, lighter weight torquetransmission coupling which does not need an electronic controller ofhigh production cost and whose constitution is relatively simple, sothat the cost of production of the coupling is lower. Of course, anelectronic controller may be employed in conjunction with the torquecoupling differential assembly described herein, whereby the fluid pumpis eliminated in favor of an electronic controller or other suitabletype of controller for the clutch mechanism or other torque transfermechanism.

While the foregoing invention has been shown and described withreference to a specific arrangement and design, it will be understood bythose of skill in the art that various changes in form and detail may bemade therein without departing from the spirit and scope of thisinvention.

I claim:
 1. A torque transmission coupling for an axle of afour-wheel-drive vehicle, said torque transmission coupling comprising:atwo-piece differential case having a first piece and a second piece; aring gear secured to and transmitting an input torque to said firstpiece; at least one pinion and side gear assembly secured to andreceiving an output torque from said second piece; a multi-disc clutchmechanism adapted to transmit torque from said first piece to saidsecond piece when said multi-disc clutch mechanism is in an engagedposition, a first set of clutch plates being secured to said first pieceand a second set of clutch plates being secured to said second piece;and a speed sensitive pump actuation system responsive to a relativerotation between said first piece and said second piece to therebyactuate said multi-disc clutch mechanism, wherein both said speedsensitive pump actuation system and said clutch mechanism are disposedwithin said differential case enclosing a differential assembly.
 2. Thetorque transmission coupling according to claim 1, wherein said ringgear input member is fastened to an outer differential housing assemblythat encapsulates at least a portion of said second piece of saidtwo-piece differential.
 3. The torque transmission coupling according toclaim 1, wherein said differential case output member drives at leastone pinion and side gear assembly.
 4. The torque transmission couplingaccording to claim 1, further comprising a bearing assembly providingrelative rotation between said first piece and said second piece.
 5. Thetorque transmission coupling according to claim 1, wherein said secondpiece of said two-piece differential case is formed as a cup-shapedmember.
 6. The torque transmission coupling according to claim 1,wherein said clutch mechanism is coaxially arranged with respect to arotational axis of said output member.
 7. The torque transmissioncoupling according to claim 1, wherein said speed sensitive pumpactuation system comprises a gerotor pump delivering pressurized fluidto said friction clutch mechanism.
 8. The torque transmission couplingaccording to claim 7, wherein said at least one pinion and side gearassembly comprises a planetary differential assembly selectivelytransmitting torque to a pair of coaxially arranged axles, and whereinsaid gerotor pump is coaxially disposed with respect to said axles. 9.The torque transmission coupling according to claim 1, wherein saidclutch mechanism comprises at least one input friction platerotationally fixed with respect to said first piece and at least oneoutput friction plate rotationally fixed with respect to said secondpiece, said input friction plate adapted to contact said output frictionplate when actuated by said speed sensitive pump actuation system. 10.The torque transmission coupling according to claim 1, wherein saidspeed sensitive actuation system comprises:a first rotary member rotatedtogether with said first piece; and a second rotary member which isprovided on said second piece facing said first rotary member so as tobe rotated together with said second piece; wherein pressure of anenclosed oil is increased by the relative rotation between said firstrotary member and said second rotary member during said relativerotation so that said oil is supplied to said clutch mechanism.